Various forms of syringe pump are commercially available and have been proposed in the patent literature. Examples of such patent literature include our own patents, such as U.S. Pat. Nos. 4,741,732 Crankshaw et al and 5,034,994 Crankshaw et al.
A head switch which operates when the drive head or pusher makes contact with the plunger flange of the syringe is required to permit:
(1) Sizing of syringes by identifying correct engagement of the drive head in the empty and filled positions to permit calculation of the internal volume of the syringe from data entered by the operator;
(2) Identification of operator interference with the drive head;
(3) Detection of separation of the syringe plunger from the drive head, which may occur with spontaneous delivery of fluid, or siphoning, from the syringe in excess of that caused by the pusher;
(4) Disengagement of the drive head and hence depressurization of the contents of the syringe prior to the sounding of an occlusion alarm.
In each of the above patents, a conventional microswitch with dual contacts was utilized. While such switches are reliable, it is necessary to incorporate a small lever to reduce the actuating force to the 10 to 20 gram range. A force above 10 gram is necessary to avoid spurious actuation if the position of the syringe pump is changed suddenly. On the other hand the actuating force must be less than 20 gram so that the plunger of a small syringe is not operated by the switch before contact has been sensed. Even with the smallest switch available, the lever must travel an excessive distance before the switch operates. This leads to unacceptable delays in sensing the contact or the release of the pusher with the drive head. A further disadvantage of a microswitch is the difficulty of rendering it waterproof without considerably increasing the force necessary to operate it.